The present invention relates to ultraviolet ray curable ink which makes recording image on flexible (soft) material possible.
The present invention also relates to an ink composition and a process for preparing a printed matter by ink jet printer in which the ink composition is used. More specifically, the present invention relates to an ink composition cured by irradiating with ultraviolet ray and a process for preparing a printed matter using the ink composition.
Conventionally, various printing methods in which ultraviolet ray curable ink is used are known.
Ultraviolet ray curable ink has been attracting attention in recent years from the viewpoint of high hardness of cured film, strong adhesion to substrate, excellent fastness properties such as scratch resistance, weather resistance and peeling resistance and that image can be recorded on all kinds of base materials and organic solvents are not required.
On the other hand, however, because the cured film of the ultraviolet ray curable ink is so hard, the ultraviolet ray curable ink has a disadvantage of breaking due to lack of conformity when image is recorded on a flexible (soft) material such as film or metal foil.
Also, curing by ultraviolet ray a composition containing (meth)acrylate having a (meth)acryloyl group as a reactive prepolymer or reactive diluent has been attempted (JP-A-7-330835). When the fast curable (meth)acrylate composition of low viscosity disclosed in the publication is used for a flexible material such as film or metal foil as an ultraviolet ray curable ink, the cured film is cracked and peeled off of the material by bending.
As mentioned above, ultraviolet ray curable ink which is excellent in conformity, of which the cured film does not break even when recording is conducted on a flexible material, and also superior in scratch resistance and adhesion does not exist.
Also, recording methods utilizing ink jet printing have been widely used.
Recently the recording medium for ink jet printing is not limited to paper and image recording is conducted using various materials such as film, plastics, ceramics, metal and fabric. However, when liquid ink is used to print an image on such materials, a clear image cannot be formed as blurring occurs.
In order to solve this problem, a method of forming an ink receiving layer on the recording face of the recording material by treating in advance is suggested. However, pre-treatment is required when forming the ink receiving layer on the recording face and post-treatment of removing the ink receiving layer becomes necessary and so the method is extremely inefficient.
To solve the above problem, a printing method in which ink jet printing is conducted directly on a recording face without forming ink receiving layer by using hot melt ink has been developed. The hot melt ink used in this printing method is obtained by dissolving a coloring agent in a solvent prepared by heating and melting a vehicle which is solid (semi-solid) at room temperature. However, the biggest problem with hot melt ink is that the image formed by the hot melt ink lacks scratch resistance as wax is used as the main component of the ink.
Further, ultraviolet ray curable ink for ink jet which can produce a cured film excellent in scratch resistance even from liquid ink by irradiating with ultraviolet ray after printing has been developed. However, the ultraviolet ray curable ink has problems such as blurring occurring during the period from printing to curing by irradiating with ultraviolet ray and limited ink viscosity for stable discharge.
In order to solve these problems, JP-A-2-311569 suggests a method of printing by heating the ink prepared from photo-polymerizable monomers, thermosoftening resin and photoinitiator by decreasing the viscosity. However, in this method, the thermosoftening resin must be dispersed in the photo-polymerizable monomers and an organic solvent is used. The organic solvent evaporates when ink is heated and thus there is the problem of smell and adverse effect on health. When the thermosoftening resin is dispersed in the photo-polymerizable monomers without using any organic solvent, homogeneous dispersion tends to be difficult and after curing of ink, uniform cured film cannot be obtained.
Also, to adjust the viscosity of ink so that the ink can be discharged from the printer head, including a monomer component having a relatively low viscosity which is called a reactive diluent is possible but the cured film obtained lacks flexibility and the adhesion to materials is poor. In particular, in the case of using ultraviolet ray curable ink containing a large amount of coloring agent which is used for coloring, the exposed ultraviolet ray is difficult to reach the inside of ink and forming a firm cured film was extremely difficult.